re: Fuel Chat

On Fri, 27 Nov 1998, Sid Jones wrote:
> >On Fri, 27 Nov 1998, Sean Bayan Schoonmaker wrote:
> >executive summary: fusion fuel is not an issue. reaction mass is
probably 
> >an issue.
> >the question is, how are you going to turn this energy into thrust?
if
> >you have grav drives or photon drives, then it is complicated but
just
> >requires some machinery. 
> 
> Right.  So if you're using a 'reactionless' drive, you need very
little
> fuel with either fission or fusion reactors to power the drive.
> 
> >if you are going to use a reaction drive (ie a
> >nuclear rocket), you will need reaction mass. you may well need a lot
of
> >reaction mass, depending on how fast you can accelerate it. if you go
for
> >boiling water with the fusion energy and venting it into space, you
will
> >need an immense amount, and will need to replenish frequently - icy
> >comets, asteroids, planetary ring bodies, etc, are possible sources.
if
> >you are using a super-high-velocity ion drive, then your reaction
mass
> >needs will be more modest.
> 
> ummm, from what I've read, a fusion rocket produces thrust in a two
part
> process:  first the frozen fuel (H or an He-Deuterium mix depending on
> the type of reaction you're using) pellet drops into a magnetic
> confinement chamber.	Second, your fusion reactor powers up a laser or
> particle beam which zaps the fuel pellet and produces a fusion
explosion
> that the confinement chamber channels out the back end of the ship as
> thrust.

oooh, *that* kind of fusion drive. right. there are two types of fusion
drive, which for want of a better term i will call internal combustion
and
external combustion.

the type you describe is external combustion: like a chemical rocket,
the
products of the heating process are expelled to generate thrust. there
are
two subtypes of this: the Orion type, where nuclear bombs are exploded
behind a big shock-absorbing plate (as used by the humans in Footfall),
and the Daedalus type, where fuel pellets are compressed by lasers to
start fusion (inertiall confinement fusion, i think this is called). the
internal combustion drive generates power which is then used to run a
motor; this is somewhate like the nuclear fission rockets that have been
proposed, as in the NERVA study in the 60s. there are again two ways of
doing this: you can run a reaction fluid in contact with your reactor to
superheat it, producing a jet, or you can collect the energy as
electricity, and use that to power an ion drive or a thermal-expansion
rocket using an electric heater (no, not like in a kettle; a maser or
something.

>  This kind of fusion pulse drive was used in Pournelle and
> Niven's _Footfall_ by the aliens.

all i remeber is that they had a ramscoop at the start and then got rid
of
it. i suppose they could have been collecting hydrogen with the scoop,
making pellets and feeding those into the drive.

> Sources:  Advanced Propulsion Study, Robert Forward (incidentally
> inventor of the Forward Mass Detector), Hughes Research Labs 1987.
> 
> The Future of Flight, L. Myrabo and D. Ing, 1985.

well, i have a PDF (sorry!) on gas-core nuclear rockets, which are
roughly
comparable to contact-heating internal-combustion fusion rockets, on me
website:

users.ox.ac.uk/~univ0938/gzg/rockets/gcnr.pdf

i'm not sure who it's by; i can't read pdf on this machine ...

> What the heck is a 'photon' drive?  How does this produce thrust?

elementary, my dear Schrodinger! photons have momentum, due to ... er
...
quantum things. if you shoot photons in one direction, you get a force
on
your photon emitter. at the power outputs in everyday life, the force is
quite small, but if you have a really, really big emitter, the momentum
change of all those photons shooting off in one direction imparts a
momentum change to the shooter. a cousin of the solar sail.

hope (a) this is right (b) this helps.

Tom